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Coronavirus Antibody Tests: Facts, Myths and Everything You Need To Know

Coronavirus Antibody Tests: Facts, Myths and Everything You Need To Know
June 05
16:59 2020

By Sumathi Reddy

The clamor for antibody tests is mounting. The blood tests are designed to detect whether people have been previously infected with the coronavirus and have developed antibodies to it. People who were sick but never got a Covid-19 test want to see if they did have the virus—in hopes that they may have some protection from future infection. Public health authorities are deploying the tests to help determine how widely the virus has spread. And businesses and governments hope the tests can help open up the economy and get employees back to the workplace.

But the tests have suffered from accuracy concerns. For weeks experts warned the tests weren’t reliable and urged individuals not to seek them. The Food and Drug Administration initially didn’t require manufacturers to get authorization from the agency and the quality of the tests was inconsistent. On May 4 the FDA announced manufacturers had to submit validation data and apply for emergency-use authorization to sell a test. So far, only about 15 tests have received that distinction. And the Centers for Disease Control and Prevention warns that antibody test results shouldn’t be used to make decisions about letting people gather in settings such as schools, dorms or offices.

Other questions surround the significance of the antibodies themselves. Though experts say there is now evidence that the presence of antibodies results in some immunity to the virus that causes Covid-19, there are still many unknowns, such as what level of antibodies is needed for protection and how long any immunity lasts.

For a closer look at all of these issues, we asked experts what we know—and don’t know—about the tests.

The Basics

What is an antibody test?

Antibody tests sample your blood to see if your body has produced antibodies. These tests don’t tell you if you have an active infection. Instead, they can determine whether you had a previous infection.

How is that different from a diagnostic test?

Diagnostic tests tell you whether you are currently infected with Covid-19. Typically, a swab is used that pushes far back into the nose to get a mucus sample. The test can detect genetic material from the virus, says David Walt, a professor of pathology at Harvard Medical School and Brigham and Women’s Hospital in Boston.

What is an antibody?

Any antibody is a protein produced by the immune system, designed to bind to particular proteins on the virus. Once the antibodies bind with the virus proteins, they ideally trigger a process to neutralize the virus and remove it from the body.

If I have antibodies to the virus that causes Covid-19, does that mean I’m protected from getting it again? If so, for how long?

Experts say recent studies have concluded that antibodies provide some level of protection or immunity from getting the virus again. What remains unknown is how long that protection lasts and what level of antibodies is needed for immunity.

Experts say based on similar viruses, the assumption is an adequate level of antibodies could result in immunity lasting for months up to a year or two but more studies need to be done.

Most antibody studies were conducted in severely ill people hospitalized for Covid-19, with only a few studies on those with more minor illness, says Amy Karger, an assistant professor in laboratory medicine and pathology at the University of Minnesota Medical School.

A not-yet-published study from Mount Sinai researchers found that people who have Covid-19 and have mild symptoms still produce robust levels of antibodies.

What is the value of antibody tests right now?

From a public-health perspective, experts say, the tests are crucial for determining the country’s and local communities’ rate of infection of the virus, and the true mortality rate. On an individual level, the tests can determine whether a person had an infection of the virus that causes Covid-19.

The CDC warns that the tests shouldn’t be used at this point for decisions, such as when employees return to the workplace.

But experts like Joshua Weitz, professor of biological sciences and director of the interdisciplinary quantitative biosciences graduate program at Georgia Tech, say planning for how to best use information from high-quality serological tests to reduce chains of transmission should start now.

“If companies and individuals do decide to act I would hope they do so knowing they should revisit their decisions with follow-up testing every few months,” says Dr. Weitz.

Gauging Quality

Where can I get an antibody test?

The FDA has approved about 15 tests and the numbers are growing. None of the approved tests can be conducted in a physician’s office, but doctors can send blood samples to labs to process the test results.

Currently large laboratory chains like Quest Diagnostics and LabCorp are conducting tests. Consumers can request a test through Quest from their primary-care physician or they can sign up for a test themselves directly through the company’s website.

Some experts expect that over-the-counter antibody tests may be available as soon as the late summer or fall but say that such tests—which could use blood from finger pricks or saliva—aren’t as accurate as laboratory tests that use drawn blood.

How reliable are antibody tests?

The quality of antibody tests improved a lot after the FDA tightened regulations in early May, says Dr. Karger. Manufacturers that couldn’t provide validation for their products within 10 days had to withdraw from the marketplace. “This removed some of the questionable products and built trust in terms of accuracy,” says Dr. Karger.

Testing companies’ data is online and can be reviewed. A couple of independent studies have also evaluated some of the antibody tests and verified their results.

“We feel much better,” says Kelly Wroblewski, director of infectious diseases for the Association of Public Health Laboratories. “The tests definitely look pretty good from a performance perspective. Some of them are obviously performing somewhat better than others but all of them perform reasonably well.”

But Gigi Gronvall, a senior scholar at the Johns Hopkins Center for Health Security, notes there is still a paucity of independent evaluation of antibody tests. “We are going primarily based on the manufacturers’ assessment of their accuracy,” says Dr. Gronvall.

Experts are eagerly awaiting a federal initiative to independently verify the tests, headed up by the National Cancer Institute, National Institutes of Health, and the CDC.

How can I tell if an antibody test is reliable?

Dr. Karger says she tends to recommend the larger manufacturers, such as Abbott, Roche, Siemens Healthcare Diagnostics and Ortho Clinical Diagnostics, given their track records with high-quality clinical lab testing.

Since the FDA tightened regulations, some companies have withdrawn their tests from the market. But many others are still awaiting review by the FDA, leaving some tests of dubious quality on the market. As a result experts recommend sticking with tests that have already been reviewed and approved.

Look for a test with a high specificity, of at least 99% and ideally 99.5% or higher. This ensures that there is less chance of getting a false positive. The FDA lists this information on its website.

Dr. Gronvall suggests that people who get an antibody test hold on to it because its accuracy may change over time. “All of the numbers are going to change as the tests are used against more samples because that’s what’s going to make it more accurate,” she says.

What are the different kinds of antibody tests?

There are four main types of antibody tests that vary in what they tell you, says Dr. Gronvall.

Rapid diagnostic tests can deliver results in about 10 to 30 minutes. The minus: They detect only the presence or absence of antibodies against the virus but don’t give the amount or titer of antibodies. The tests aren’t very sensitive so they may produce high rates of false negatives.

There are currently three FDA-approved rapid tests but two of the three tests are approved for use with a blood draw only. Chembio’s test is the only rapid test approved for use with fingerstick blood but it must be conducted in a moderate or high-complexity laboratory.

The most common type of antibody tests currently used are ELISA (enzyme-linked immunosorbent assay) and CLIA (chemiluminescent immunoassay) tests. Both are laboratory tests done from blood drawn and measure the level of antibodies. But Dr. Karger says most tests aren’t reporting such information to consumers because it’s not yet clear what the numbers mean.

The final type of antibody test is a neutralizing assay which is a highly complex and expensive method usually only used in a research setting. Such tests only measure one type of antibody—the neutralizing antibodies that block the virus and are thought to confer protective immunity.

What’s the risk of a high false positive or false negative rate?

Test makers try to balance specificity with sensitivity. A highly specific test will exclude false positives but might be less sensitive and miss some positives. If a test is very sensitive, it may pick up a signal in people not really infected.

Experts are especially concerned about the possibility of false-positive results with Covid-19, because people might think they have immunity to the virus when they don’t. Many companies focused on minimizing false positives.

Now, there are growing concerns about sensitivity, or that the tests may miss some people with lower levels of antibodies and produce false negatives.

Another challenge is that the tests may measure antibodies produced for other coronaviruses, which don’t reflect infection with Covid-19. There are several other common coronaviruses that are seasonal and cause the common cold.

Producing Antibodies

How long after infection does the body start producing antibodies? How long do we have them?

The body starts producing antibodies about a week after infection, and nearly everyone has antibodies in their blood 10 to 14 days after symptoms start. It is unknown how long the antibodies will remain, but one study found that for another deadly coronavirus, severe acute respiratory syndrome, or SARS, the antibodies faded after a couple of years, he says.

Are there different types of antibodies?

Yes. In a typical immune response, the first class of antibodies produced is called IgM, or immunoglobulin M. Next, the body produces IgG, which is better able to recognize and target the specific virus. The body also produces IgA, which is usually found in higher amounts on mucus-membrane surfaces.

IgM is transient and typically isn’t detectable after a few months. IgG is detectable for much longer and is probably the most useful to measure.

Some tests, such as the rapid tests, detect IgM and IgG separately with separate lines, says Dr. Karger. The ELISA and CLIA methods either measure IgG only or they measure the total amount of antibodies, which would include IgG, IgM, and IgA.

Researchers are trying to identify the antibodies that are most important, or the neutralizing antibodies. “When you form antibodies to a virus they stick to different parts of the virus and a lot of them are just decorations on the virus. They’re not helping you actually fight off the virus or blocking it from getting into cells,” says Dr. Gronvall.

I am young and healthy. Should I just expose myself to the virus so that I can eventually get an antibody test and resume my normal life?

Experts recommend against doing this. You never know what kind of reaction you may have, and even young and healthy people have died. The goal is to take pressure off the health-care system, so exposing yourself intentionally—and exposing those around you—is foolish, they say.

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